1. Field of the Invention
The present invention relates to a process for the preparation of polysuccinimide and copoly(succinimide-aspartate) dissolved or dispersed in a supercritical fluid (SCF), such as liquid CO2 starting with an aminoacid such as L-aspartic acid.
2. Discussion of the Related Art
L-aspartic acid has been produced commercially since the 1980's via immobilized enzyme methods. The L-aspartic acid so produced mainly has been used as a component of the synthetic sweetener, N-aspartylphenylalanine methyl ester (ASPARTAME®).
In a typical production pathway, a solution of ammonium maleate is converted to fumarate via action of an immobilized enzyme, maleate isomerase, by continuous flow over an immobilized enzyme bed. Next, the solution of ammonium fumarate is treated with ammonia also by continuous flow of the solution over a bed of the immobilized enzyme, aspartase. A relatively concentrated solution of ammonium asparate is produced, which then is treated with an acid, for example nitric acid, to precipitate aspartic acid. After drying, the resultant product of the process is powdered or crystalline L-aspartic acid. Prior art that exemplifies this production pathway includes U.S. Pat. No. 4,560,653 to Sherwin and Blouin (1985), U.S. Pat. No. 5,541,090 to Sakano et al. (1996), and U.S. Pat. No. 5,741,681 to Kato et al. (1998).
In addition, nonenzymatic, chemical routes to D,L aspartic acid via treatment of maleic acid, fumaric acid, or their mixtures with ammonia at elevated temperature have been known for over 150 years (see Harada, K., Polycondensation of thermal precursors of aspartic acid. Journal of Organic Chemistry 24, 1662-1666 (1959); also, U.S. Pat. No. 5,872,285 to Mazo et al. (1999)). The above chemical routes of maleic acid and ammonia are less sterically controlled as well as less quantitative and the product is a D,L racemic mixture. Although the non-enzymatic routines are significantly less quantitative than the enzymatic syntheses of aspartic acid, possibilities of continuous processes and recycling of reactants and by-products via chemical routes are envisioned.
Polymerization and copolymerization of L-aspartic acid alone or with other comonomers is known. As reviewed in U.S. Pat. No. 5,981,691 to Sikes (1999), synthetic work with polyamino acids, beginning with the homopolymer of aspartic acid, dates to the mid 1800's and has continued to the present. Interest in polyaspartates and related molecules increased in the mid 1980's as awareness of the commercial potential of these molecules grew. Particular attention has been paid to biodegradable and environmentally compatible polyaspartates for commodity uses such as detergent additives and superabsorbent materials in disposable diapers, although numerous other uses have been contemplated, ranging from water-treatment additives for control of scale and corrosion to anti-tartar agents in toothpastes.
There have been some teachings of producing copolymers of succinimide and aspartic acid or aspartate via thermal polymerization of maleic acid plus ammonia or ammonia compounds. For example, U.S. Pat. No. 5,548,036 to Kroner et al. (1996) taught that polymerization at less than 140° C. resulted in aspartic acid residue-containing polysuccinimides. However, the reason that some aspartic acid residues persisted in the product polymers was that the temperatures of polymerization were too low to drive the reaction to completion, leading to inefficient processes.
JP 8277329 (1996) to Tomida exemplified the thermal polymerization of potassium aspartate in the presence of 5 mole % and 30 mole % phosphoric acid. The purpose of the phosphoric acid was stated, in the above patent, to serve as a catalyst so that molecules of higher molecular weight might be produced. However, the products of the reaction were of a lower molecular weight than were produced in the absence of the phosphoric acid, indicating that there was no catalytic effect. There was no mention of producing copolymers of aspartate and succinimide; rather, there was mention of producing only homopolymers of polyaspartate. In fact, addition of phosphoric acid in this fashion to form a slurry or intimate mixture with the powder of potassium aspartate, is actually counterproductive to formation of copolymers containing succinimide and aspartic acid residue units, or to formation of the condensation amide bonds of the polymers in general. That is, although the phosphoric acid may act to generate some fraction of residues as aspartic acid, it also results in the occurrence of substantial amounts of phosphate anion in the slurry of mixture. Upon drying to form the salt of the intimate mixture, such anions bind ionically with the positively charged amine groups of aspartic acid and aspartate residues, blocking them from the polymerization reaction, thus resulting in polymers of lower molecular weight in lower yield.
Earlier, U.S. Pat. No. 5,371,180 to Groth et al. (1994) had demonstrated production of copolymers of succinimide and aspartate by thermal treatment of maleic acid plus ammonium compounds in the presence of alkaline carbonates. The invention involved an alkaline, ring-opening environment of polymerization such that some of the polymeric succinimide residues would be converted to the ring-opened, aspartate form. For this reason, only alkaline carbonates were taught and there was no mention of cations functioning themselves in any way to prevent imide formation.
More recently, U.S. Pat. No. 5,936,121 to Gelosa et al. (1999) taught formation of oligomers (Mw<1000) of aspartate having chain-terminating residues of unsaturated dicarboxylic compounds such as maleic and acrylic acids. These aspartic-rich compounds were formed via thermal condensation of mixtures of sodium salts of maleic acid plus ammonium/sodium maleic salts that were dried from solutions of ammonium maleate to which NaOH had been added. They were producing compounds to sequester alkaline-earth metals. In addition, the compounds were shown to be nontoxic and biodegradable by virtue of their aspartic acid composition. Moreover, the compounds retained their biodegradability by virtue of their very low Mw, notwithstanding the presence of the chain-terminating residues, which when polymerized with themselves to sizes about the oligomeric size, resulted in non-degradable polymers.
A number of reports and patents in the area of polyaspartics (i.e., poly(aspartic acid) or polyaspartate), polysuccinimides, and their derivatives have appeared more recently. Notable among these, for example, there have been disclosures of novel superabsorbents (U.S. Pat. No. 5,955,549 to Chang and Swift, 1999; U.S. Pat. No. 6,027,804 to Chou et al., 2000), dye-leveling agents for textiles (U.S. Pat. No. 5,902,357 to Riegels et al., 1999), and solvent-free synthesis of sulfhydryl-containing corrosion and scale inhibitors (EP 0 980 883 to Oda, 2000). There also has been teaching of dye-transfer inhibitors prepared by nucleophilic addition of amino compounds to polysuccinimide suspended in water (U.S. Pat. No. 5,639,832 to Kroner et al., 1997), which reactions are inefficient due to the marked insolubility of polysuccinimide in water.
U.S. Pat. No. 5,981,691 to Sikes et al purportedly introduced the concept of mixed amide-imide, water-soluble copolymers of aspartate and succinimide for a variety of uses. The concept therein was that a monocationic salt of aspartate when formed into a dry mixture with aspartic acid could be thermally polymerized to produce the water-soluble copoly(aspartate, succinimide). The theory was that the aspartic acid comonomer when polymerized led to succinimide residues in the product polymer and the monosodium aspartate comonomer led to aspartate residues in the product polymer. It was not recognized that merely providing the comonomers was not sufficient to obtain true copolymers and that certain other conditions were necessary to avoid obtaining primarily mixtures of polyaspartate and polysuccinimide copolymers. In U.S. Pat. No. 5,981,691, the comonomeric mixtures were formed from an aqueous slurry of aspartic acid, adjusted to specific values of pH, followed by drying. There was no teaching of use of solutions of ammonium aspartate or any other decomposable cation plus NaOH, or other forms of sodium or other cations, for generation of comonomeric compositions of aspartic acid and salts of aspartate. Thus, although some of the U.S. Pat. No. 5,981,691 examples obtain products containing some copolymer in mixture with other products, particularly homopolymers, as discussed in the Summary of the Invention below, the theory that true copolymers could be obtained merely by providing the comonomers in the manner taught in U.S. Pat. No. 5,981,691 was not fully realized.
Thus, to date, there have been no successful disclosures of water-soluble or wettable, mixed amide/imide polyamino acids such as copolymers of aspartate and succinimide, related imide-containing polyamino acids, polysuccinimide or derivatives thereof.